Wide-band amplifier



c. LE COMTE 2,676,215

WIDE-BAND AMPLIFIER Filed June 21, 1951 April 20, 1954 INVENTOR CORSTIAN LE co TE AGENT Patented Apr. 20 1954 UNITED STATE OFFICE WIDE-BAND AMPLIFIER Application June 21, 1951, Serial No. 232,728

Claims priority, application NetheilanclS July 12, 1950 3 Claims. 1

This invention relates to wide-band amplifiers such as are used for amplifying videoor radarpulse signals (in general step-functions) and the amplification of which is required to be substantially independent of the frequency over a wide frequeney range, for example from 25 cycles per sec. to several megaoycles per sec.

Known wide-band amplifiers comprisiiog a cascade-pair cf resistance-coupled amplifier-tubes entail the dificulty that, due to parasitic capacities (notably the parasitic anode-capacity of the first tube and the grid-capacity of the second tube), the higher frequencies of the signal to be amplified are amplified to a lesser degree than the lower frequencies. In order to counteract the influence of the parisitic anode capacity of the first tube, the anode circuit of this tube usually comprises an impeclance consisting of the .seriesconnection of a resistor and an inductance (shunt peaking oeil), which inductance compensates for the decrease in amplification due to the parasitic anode-capacity at the highest frequenoy to be amplified. If the phase curve of the amplifier in the neighbourhood of the higher frequencies cloes not meet the requirements it may be improved by connecting, for example, a small capacitor in parallel With the said resistor.

A coi1pling capacitor capable of passing the whole frequency range, inclusive of the lowest frequencies to be amplified, is required for supplying the voltage across the anode impedance of the first tube to the grid-circuit of the second tube. If desired, a small inductance (series peaking coil) may be connected in series With the coupling capacitor to compensate for the decrease in amplification at the highest frequencies due to the parasitic grid-capacity of the second tube.

Although the shunt peaking coil and the series peakng coil yield a considerable improvement in the response curve, the detrimental effect of the said parasitic capacities is still present to an undesirable extent in the known circuits.

The present invention provides a circuitarrangement in which the natural capacity of the electrodes of the coupling capacitor With respect to earth, which in the known circuits described above is in parallel With the detrimental anodecapacity of the first tube or With the grid-capacity of the second tube, becomes operative parallel to the anode resistor, thus aise improving the phase curve.

According to the invention provision is made of a winding which is inductively coupled to the anode inductance of the first tube and has one 2 end conductively connected to the grid of the second amplifier tube whilst the other end is connected through the coupling capacitor to the series-connection cf resistor and inductance at a point located between this resistor and inductance.

In order that the invention may be readily carried into efect, two examples Will now be described With reference to the accompanying drawmg.

Fig. 1 thereof shows a circuit-arrangementaccording to the invention in which the voltage supplied to the second tube is equal to that across the whole anode impedance of the first tube.

Fig. 2 shows an embodiment of the invention wherein the anode circuit of the first tube comprises 9. filter-network and part of the voltage is supplied through this network to the second tube.

In the circuit-arrangement shown in Fig. 1, the reference numeral l denotes a first amplifier tube and the numeral 2 a second amplifier tube et a wide-band amplifier for signal amplification over a very wide frequenoy range, for example 25 cycles per sec. to 10 to 20 megacycles per sec. The anode circuit of tube l comprises a resistor 3 connected in series With a small inductance 4 (shunt peaking coi!) such that the voltage produced across this series-connection is practically frequency-independent throughout the frequency range of the oscillations to be amplified despite the parasitic anode-capacity 5 of tube l.

In the known arrangements using the shunt peaklng coil, voltage is supplied to the grid circuit cf tube 2 front the series-connection 3, 4 through a coupling capacitor 6 (for example one of 50,000 pf.) In this case, the parasitic capacitics 7 and 8 of the electrodes of this coupling condenser 6 With respect to earth will be located in parallel With the parasitic anode capacity 5 01 tube I and the parasitic grid capacity 9 of tube 2 respectively, thus adversely affecting the amplification curve. According to the invention this is avoided by providing a winding [0 which is inductively coupled to the inductance 4 and has one end conductively connected to the grid of tube 2 while its other end is connected through the coupling capacitor 5 to a point between the resistor 3 and the inductance 4.

The voltage produced across resistor 3 is increased by the voltage across the winding H! and subsequently supplied to the grid 01 tube 2. With a transformation ratio 01 1:1 for the windings 4 and 20 the voltage supplied to tube 2 is consequently equal to the voltage across the seriesconnection of resistor 3 and inductance 4. In

this case, however, the parasitic capacities 1 and. 8 of the electrodes of the coupling capacitor 5 With respect to earth are no longer in parallel With the detrimental parasitic capacity 5, but With the resistor 3 With the result that the phase curve in the neighbourhood of the highest frequenoy is improved if the said capacity has a suitable value. The amplifier then appears to have a 20 to 30% greater band-vviclth with the saine amplification as that obtained in the absence of the invention.

If the coupling factor betveen the windings 4 and i@ lies between 0.75 and 0.9, for example approximately 0.8, a urther improvement of the amplification curve and phase curve is obtainable inter alia because the leakage inductance produced in the grid-circuit of tube "2 W111 aot as a series peaking oeil and Wi1l thus reduce the influence of the parasitic capacity S.

In Fig. 2, in which the same reference numerals are used as in Fig. 1, the anode circuit of the tube i comprises a filter-network consisting of the said resistor 3 With a low parallel capacity, incluctance 6%, and a further ductance H connected in series with the inductance 4. The ends oi induetance il have a eapacity 5 and :53 respectively with respect to earth. In a known circuitarrangement comprising two induetance such as i i and 4 in series, their jonction is linked through the coupling capacitor to the grid tube 52. In this way, however, the capacity E! is unnecessarily increased sinoe in this case not only the parasitio grid capacity & of tube 2 but also the parasitic capacities '5 and 3 of the electrodes or the coupling capacitor contribute to the capacity 9'.

According to the invention provision is again made of a winding i which is inductively ooupled to the inductance and has one end oonductively connected to the gricl of tube 2, Whiie its other end is connected through coupling capacitor 5 to a point between the inductance =i and the resistor 3. In this case, the junction of the inductances H and 3 bas, with respect to earth, a capacity 9' which, if the inductances and. li! are closely coupled, corresponds to the parasitic grid capacity 9 of tube 2, the parasitic capacities E and 8 of the electrodes of the ooupling capaeitor 6 with respect to earth contributing to the initially-required capacity parallel to the resistor 3.

A further improvement is obtainable by conmeeting the grid-leak resistor 03? tube 2, not, as

shown in the drawing, in parallel with the grid capacitance 9 of tube 2 but in parallcl with the capacitance 3, with the result that this gridcapacitance 9 may also be reduced.

What I daim is:

1. A wide-band amplifier for ampiiying signals in a frequency range extencling from a point in the audio frequency banc. to at least approximately one megacycle comprising first and second ampiitying stages each including an electron diseharge tube having a cathode, 3, gris, and an anode and circuits thereior, means to saisi signals to the grid of saie1 first stage, an impedance in the anode circuit of the i1rst stage Which is substantially frequency-indepn ent within said range whioh incuac.es resistor in series with a shunt peaking c oeil having a peak response compensang decrease in amplification in the higher portion of said signal range, a capaoitor for :0 ing irequencies in said range connecter net the junction of the seriesconnectcd eoii sistor and the grid o:i the second 5* go, capacitor seing subject to arasitic capaeitanc9 reiative to ground, a Winxiing interpose i betwe the gris! 01 said second stage and sai-i capa winding being incluctively coupled to 0011, SL{, Winding and said coil having a coup n factor relative impedance vaines mineci to compensate for the decrease in ampl-fi. cation in the higher frequenoy portion o saiol range due to saki parasitc oapaoitance, and means 'to derive the ampli-lied signals from the anode circuit of said second stage.

2. An amplifier as claimed in claim 1, characterized in that the coupling factor between the inductance and the winding has a value between 0.75 and 0.9.

3. An amplifier as claimed in claim 1, char-aeterized in that the anode circuit of the first tube includes, in series with the said inductance, a second inductance Which together therewith and with theparasitic capacities and the resistor constitutes a filter-network.

Beferences Gited in the file of this patent UNIIED STATES PATENTS Achenbach Jan. 1, 1952 

